1. Field of the Invention
The invention relates to nuclear fuel assemblies and more precisely to hairpin springs for supporting the fuel rods in spacing grids belonging to such assemblies.
2. Description of the Prior Art
Fuel assemblies used at the present time in light water cooled and moderated reactors include a bundle of fuel rods (each formed by a stack of fuel pellets in a sealed sheath) held in a regular pattern in a structure. The structure includes two end nozzles joined together by tie rods replacing the fuel rods at some nodal points of the pattern and grids spaced apart along the assembly and formed by plates disposed in two mutually orthogonal directions so as to define cells most of which are traversed by fuel rods. Each cell is provided with means bearing on and supporting the fuel rod which passes through the cell.
The supporting means may be formed as parts cut out in the plates, forming lugs which frictionally bear against the rods. Since they must be of a material which retains satisfactory mechanical properties under irradiation, the whole grid must then be of a material having a low neutron capture cross-section, for instance one of the zirconium based alloys. It has consequently been found advantageous to use springs fixed to the plates rather than springs integral with the plates. Then the springs may be of a material different from that of the plates (for example "Inconel" instead of a zirconium based alloy). A grid of the latter type is described for example in French Pat. No. 2,474,229 to which reference may be made.
The springs are typically in the shape of a hairpin with two legs. Often, several types of spring need be provided in the same grid, because the grid has lateral as well as internal plates and often some cells are occupied by elements other than fuel rods, for example by tie rods also forming guide tubes. A frequently used type of spring has two identical legs, whereas another type of spring has different legs. Each spring is inserted, then secured to a plate by welding the two legs to each other in well-defined zones, through windows formed in the plate.
Positioning each spring is delicate: it is necessary to locate it accurately so as not to clamp it on the plate during welding and to avoid contamination by welding projections. During welding through an upper window, it may happen that the spring is fixed in an angular position such that the legs do not face the lower window. A defect may also result from an accidental mutual shifting of the two legs with respect to each other or from lack of contact during welding.
Such defects cannot be accepted in a grid. An excessive rod clamping force, which could cause local destruction of the sheath during differential heat expansion, and an insufficient force which would result in vibrations during use and wear of the sheath should be avoided.
Spring positioning devices are already known which considerably reduce the risk of faulty positioning of the springs with respect to the grid plate which receives it (U.S. Pat. No. 4,646,431 to Vere et al). But the risk is not completely excluded, particularly if springs are distorted.